The present invention relates to a peripheral indexable cutting insert for mounting to a tool holder under an axial angle different from zero, said insert being made of a trapezoid body and comprising a substantially planar mounting face, a substantially plane front face opposite said mounting face and side faces formed therebetween as well as at least a pair of opposite cutting edges each including a respective chip face and an adjacent clearance face, wherein the chip faces each are formed by a hollow groove shaped in the front face and the clearance faces each are formed by one of said side faces.
Peripheral indexing cutting inserts are known since long. They generally comprise a prismatic body including a planar mounting face and at least a cutting edge corresponding to the intersection line between a chip face and a clearance face along the side face of the indexable insert. The insert is secured to a tool holder such that the cutting edge performs a circle when the tool head is rotated. Typically, the insert is located under an angle with respect to the rotational axis (axial angle). This axial angle directly affects the tool life parameter and the life time of the cutting edge. A smooth cutting operation results from a relatively large axial angle.
For an axial angle different from zero and for a straight cutting edge, an accurate 90.degree.-angle can not be milled since the leading end and the lagging end of the cutting edge are not located on a common cylindrical face. Furthermore, the chip angle and clearance angle as well as the sign of the chip angle may change across the length of the cutting edge which can be detrimental for a high quality performance. However, certain angles have to be selected to obtain an optimum lifetime and to facilitate the chip removal, which angles can not be maintained across the length of the cutting edge in view of what has been discussed above. From this follows that cutting tools of the indexable insert type are not used for a peripheral finishing operation. Rather, cutting tools having helical cutting edges have been used heretobefore to fulfill the needs required. Cutting tools of this type, however, are relatively expensive when being provided with removable cutters.
EP 0 392 729 and 0 392 730 disclose peripheral indexable cutting inserts tending to avoid the drawbacks of conventional inserts. The cutting edge is curved, i.e. as an intersection line of a plane section through a cylinder. The angle of the section plane corresponds to the axial angle. This ensures that the cutting edge lies on a cylindrical face and that the tool is suited to provide an accurate angle of 90.degree..
According to a further feature of the prior art cutting tool, the chip face and the clearance face are continuously curved such that a constant chip angle or, respectively a constant clearance angle result therefrom along the cutting edge.
The advantage of the prior art tool is seen in the fact that a relatively large axial angle can be selected. Furthermore, the chip angle as well as the clearance angle can be optimized for a positive cutting geometry. However, there is a drawback that the known inserts are formed without cutting, in particular by a sintering process which itself is not particularly expensive. However, the sintering process leads to allowances not permissible, in particular the so-called concentricity is objected too. Furthermore, each different insert geometry requires a separate forming tool, even when the chip angle or the clearance angle only are slightly changed. From this follows that there is a need for a higher number of forming tools. A further drawback is encountered as sintered inserts can not be subjected to a coating process according to the PVD-method. Coating processes of this type to surface-harden the tool mostly require a preceeding grinding operation of the tool which desired inserts do not allow.